The present invention relates to a tapered roller bearing to be incorporated in a differential, transmission and the like of an automobile.
The tapered roller bearing is suitable for being loaded with radial load, axial load and a combination of such loads and has a large load bearing capacity. For this reason, tapered roller bearings are often used for supporting rotatable elements in power transmission devices (differentials, and transmissions) and the like of automobiles and construction machines.
For example, in a front-engine rear-drive automobile, engine, clutch and transmission are positioned in the front region of a car body and differential and drive shaft are positioned in the rear region of the car body; therefore, a propeller shaft is used for transmitting power therebetween. The rotary power of the engine is reduced in speed by the transmission (speed reducer) and then transmitted to the propeller shaft, through which it is fed into the differential (final speed reducer). The differential comprises a speed-reduction gear device and a differential device, wherein the speed-reduction gear device serves for rotary speed reduction and drive power increase, and particularly, in a vertical engine vehicle, the speed-reduction gear device changes the direction of transmission of driving power to right angle direction and then transmits it to a driving wheel shafts, while the differential device has the function which, when there occurs a difference in rotary speed between the right and left driving wheels, allows the two wheels to rotate on the differential basis and prevents slippage of the wheels.
FIG. 15 shows by way of example a tapered roller bearing to be incorporated in a differential as described above gear device. This tapered roller bearing comprises an outer ring 11 having a conical raceway surface 11a, an inner ring 12 having a conical raceway surface 12a and also having a retaining rib face 12b at the small diameter side of the raceway surface 12a and a cone back face rib face 12c at the large diameter side, a plurality of tapered rollers 13 disposed for rolling between the raceway surfaces 11a and 12a of the outer and inner rings 11 and 12, and a cage 14 for retaining the tapered rollers 3 at predetermined circumferential intervals.
During operation of the bearing, the tapered rollers 13 are pressed against the cone back face rib face 12c of the inner ring 12 by a resultant force received from the raceway surfaces 11aand 12a, while rolling on the raceway surfaces with their large end faces 13a contacted and guided by the cone back face rib face 12c. On the other hand, during operation of the bearing, the small end faces 13b of the tapered rollers 13 do not contact the retaining rib face 12b of the inner ring 12, so that there are slight clearances therebetween. Therefore, in the bearing producing process, as concerns the rib faces of the inner ring 12 and the end faces of the tapered roller 13; for purpose of reducing wear, grinding-finish is applied only to the cone back face rib face 12c and large end faces 13a where slide contact occurs, but not applied to the retaining rib face 12b and small end faces 13b where slide contact does not occur.
When such tapered roller bearing described above is assembled such that the assembly comprising the cage 14, plurality of tapered rollers 13 and inner ring 12 is inserted from above into the raceway surface 11a of the outer ring 11 with the retaining rib face 12b of the inner ring 12 directed downward, then, in the assembled state (initial state), the tapered rollers 13 do not sit in their normal positions on the raceway surface (because their degree of freedom with respect to the cage 14 and inner ring 12 does not allow the tapered rollers 13, when inserted, to have their attitudes fixed), so that, as shown in FIG. 16(a), the small end faces 13b come in contact with the retaining rib face 12b of the inner ring 12, with clearances .delta. defined between the large end faces 13a and the cone back face rib face 12c. When the bearing is rotated by a predetermined number of revolutions from this initial state with a thrust load Fa imposed on the bearing {FIG. 16(c)}, the tapered rollers 13 are axially moved by an amount corresponding to the clearances .delta. toward the cone back face rib face 12c until the large end faces 13a contact the cone back face rib face 12c to allow the tapered rollers 13 to settle in their normal positions {FIG. 16(b)}.
In the initial state shown in FIG. 16(a), the bearing is mounted on the mount portion of a mating device, fixed therein, pre-loaded, and normally operated, whereupon the tapered rollers 13 are axially moved toward the cone back face rib face 12c, resulting in the loss of the pre-load, making it impossible to obtain the required bearing function. Accordingly, as conventionally practiced, prior to normal operation, the bearing in the initial state shown in FIG. 16(a) is temporarily fitted in the mount portion of a mating device and is run in until the tapered rollers 13 settle in their normal positions as shown in FIG. 16(b), whereupon the bearing is fixed in the mount portion and is given a predetermined pre-load. In this case, if the clearances .delta. in the initial state increase in dimension or the range of variation thereof enlarge, or if the axial movements of the tapered rollers 13 toward the cone back face rib face 12c do not take place smoothly, the running-in time required for allowing the tapered rollers 13 to settle in their normal positions increases and so does the time required until the establishment of pre-load is completed.
Therefore, from the viewpoint of reducing the running-in time, it is desirable that the dimensions of the clearances .delta. and the range of variation thereof in the initial state shown in FIG. 16(a) be minimized; however, the conventional tapered roller bearing presents the following problem.
As shown enlarged in FIG. 17, the retaining rib face 12b of the inner ring 12 of the conventional tapered roller bearing is outwardly inclined with respect to the small end faces 13b of the tapered rollers 3 disposed on the raceway surface 12a. Therefore, because of the variation in the dimension and shape of the chamfers of the small end faces 13b (generally, the small end face of a tapered roller is as-forged, and the variation in the dimension and shape of the chamfers thereof are large, which variation in the dimension and shape of the chamfers is found not only among tapered rollers but also circumferentially of a tapered roller itself), the points of contact between the small end faces 13b and the retaining rib face 12b in the assembled state (initial state) vary. For example, if the chamfers of the small end faces 13b are as shown in solid line in the same figure, the points of contact in the initial state are P3 and P4, whereas if the chamfers of the small end faces 13b are as shown in dotted line in the same figure, the points of contact in the initial state move toward the large diameter side to P3' and P4'. When the tapered rollers 13 are axially moved toward the side associated with the large end faces and contacted at large end face thereof with the cone back face rib face, let .delta.3 be the values of the clearances between points P3 and P4, and .delta.4 be the values of the clearances between points P3' and P4', .delta.3&lt;.delta.4. This means the values of the clearances .delta. vary with the variation of the contact points caused by the variation in the chamfer dimension. For this reason, it is difficult to secure the clearances .delta. with accuracy.
Further, since the variation in the dimension and shape of the chamfers of the small end faces 13b causes the values of the clearances .delta. to vary, the variation of the clearances .delta. will inevitably increase even if the raceway groove width dimension (W') of the inner ring 12 and the length dimensions (L') of the tapered rollers are controlled with accuracy.
When the variation in the clearances .delta. caused by the variation in the dimension and shape of the chambers of the small end faces of the tapered rollers is considered with respect to a certain tapered roller bearing, the variation occurs among the plurality of assembled tapered rollers, producing a difference in the time it takes for the tapered rollers to settle in their normal positions. Therefore, the number of times for the running-in of the bearing (number of revolutions of the bearing: number of times for settlement) required till completion of settlement as a bearing is increased. Further, since the variation in the clearances .delta. occurs among bearings, there occurs among bearings a variation in the number of times for settlement. An attempt to tackle this problem by reducing the control range (dimensional tolerance) for the raceway groove width dimension (W') and length dimensions (L') and by reducing the reference value for the dimensions of the clearances .delta. would lead to increases in machining cost and control cost.
Further, normally, the control of the raceway groove width dimension (W') of the inner ring 12 is made with the end surface used as a dimensional reference, but errors are likely to build up and it is difficult to reduce the variation in the raceway groove width dimension (W'). To solve this problem, it may be contemplated to control the raceway groove width dimension (W') with the retaining rib face 12b used as a dimensional reference. However, the conventional retaining rib face 12b has an inclined shape and a difference in the raceway groove width dimension (W') occurs according to how a reference position is selected, so that it is difficult to finish the groove with dimension (W') with accuracy.
Further, conventionally, the running-in operation has been practiced with the tapered rollers retaining the rust-preventive oil initially applied thereto when shipping. However, this rust-preventive oil applied when shipping is intended mainly for rust prevention, being poor in lubricating performance. Therefore, formation of oil films between the rolling surfaces of the tapered rollers and the raceway surfaces of the inner and outer rings is insufficient, resulting in the tapered rollers sometimes failing to smoothly axially move toward the side associated with the large end face, tending to lengthen the time for running-in operation.
Further, the conventional tapered roller bearing is designed such that the position of the center of the region of contact between the rolling surface 13c of the tapered roller 13 and the raceway surfaces 12a and 11a of the inner and outer rings 12 and 11 is located in the axial center of the tapered roller 13 (the position of 1/2 of the length L'), so that there are cases where the axial movements of the tapered rollers 13 during running-in operation do not occur smoothly, tending to lengthen the running-in time.